black or white body color: inert to white to moderate light blue, green, or yellow in long and short wave. May also phosphoresce; common opal: inert to strong green or yellowish green in long and short wave, may phosphoresce; fire opal: inert to moderate greenish brown in long and short wave, may phosphoresce.[1]

The water content is usually between three and ten percent, but can be as high as twenty percent. Opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the reds against black are the most rare, whereas white and greens are the most common. These color variations are a function of growth size into the red and infrared wavelengths. Opal is Australia's national gemstone.

Precious opal

Precious opal consists of spheres of silica of fairly regular size, packed into close-packed planes which are stacked together with characteristic dimensions of several hundred nm.

Precious opal shows a variable interplay of internal colors and even though it is a mineraloid, it does have an internal structure. At micro scales precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close-packedlattice. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal.[4] It is the regularity of the sizes and the packing of these spheres that determines the quality of precious opal. Where the distance between the regularly packed planes of spheres is approximately half the wavelength of a component of visible light, the light of that wavelength may be subject to diffraction from the grating created by the stacked planes. The spacing between the planes and the orientation of planes with respect to the incident light determines the colors observed. The process can be described by Bragg's Law of diffraction.

Visible light of diffracted wavelengths cannot pass through large thicknesses of the opal. This is the basis of the optical band gap in a photonic crystal, of which opal is the best known natural example. In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during solidification. The term opalescence is commonly and erroneously used to describe this unique and beautiful phenomenon, which is correctly termed play of color. Contrarily, opalescence is correctly applied to the milky, turbid appearance of common or potch opal. Potch does not show a play of color.

The veins of opal displaying the play of color are often quite thin, and this has given rise to unusual methods of preparing the stone as a gem. An opal doublet is a thin layer of opal, backed by a swart mineral such as ironstone, basalt, or obsidian. The darker backing emphasizes the play of color, and results in a more attractive display than a lighter potch.

Combined with modern techniques of polishing, doublet opal produces similar effect of black or boulder opals at a mere fraction of the price. Doublet opal also has the added benefit of having genuine opal as the top visible and touchable layer, unlike triplet opals.

The triplet-cut opal backs the colored material with a dark backing, and then has a domed cap of clear quartz or plastic on top, which takes a high polish and acts as a protective layer for the relatively fragile opal. The top layer also acts as a magnifier, to emphasize the play of color of the opal beneath, which is often of lower quality. Triplet opals therefore have a more artificial appearance, and are not classed as precious opal.

Common opal

Besides the gemstone varieties that show a play of color, there are other kinds of common opal such as the milk opal, milky bluish to greenish (which can sometimes be of gemstone quality), resin opal which is honey-yellow with a resinous luster, wood opal which is caused by the replacement of the organic material in wood with opal,[5]menilite which is brown or grey, hyalite is a colorless glass-clear opal sometimes called Muller's Glass, geyserite, also called siliceous sinter, deposited around hot springs or geysers and diatomite or diatomaceous earth, the accumulations of diatom shells or tests.

Other varieties of opal

Fire opals are transparent to translucent opals with warm body colors yellow, orange, orange-yellow or red and they do not usually show any play-of-color, although occasionally a stone will exhibit bright green flashes. The most famous source of fire opals is the state of Querétaro in Mexico and these opals are commonly called Mexican fire opals.

Peruvian opal (also called blue opal) is a semi-opaque to opaque blue-green stone found in Peru which is often cut to include the matrix in the more opaque stones. It does not display pleochroism.

Boulder opal carving of a walrus, showing flashes of color from the exposed opal. The carving is 9 cm (3.5 inches) long.

Sources of opal

Polished opal from Yowah, Queensland, Australia

Australia produces around 97% of the world's opal. 90% is called ‘light opal’ or white and crystal opal. White makes up 60% of the opal productions but cannot be found in all of the opal fields. Crystal opal or pure hydrated silica makes up 30% of the opal produced, 8% is black and only 2% is boulder opal.[citation needed]

The town of Coober Pedy in South Australia is a major source of opal. Andamooka in South Australia is also a major producer of matrix opal, crystal opal, and black opal. Another Australian town, Lightning Ridge in New South Wales, is the main source of black opal, opal containing a predominantly dark background (dark-gray to blue-black displaying the play of color). Boulder opal consists of concretions and fracture fillings in a dark siliceous ironstone matrix. It is found sporadically in western Queensland, from Kynuna in the north, to Yowah and Koroit in the south.[6]

Multi-colored rough opal specimen from Virgin Valley, Nevada, USA

The Virgin Valley opal fields of Humboldt County in northern Nevada produce a wide variety of precious black, crystal, white, fire, and lemon opal. The black fire opal is the official gemstone of Nevada. Most of the precious opal is partial wood replacement. Miocene age opalised teeth, bones, fish, and a snake head have been found. Some of the opal has high water content and may desiccate and crack when dried. The largest black opal in the Smithsonian Institution comes from the Royal Peacock opal mine in the Virgin Valley.[citation needed]

Another source of white base opal or creamy opal in the United States is Spencer, Idaho. Spencer has an open pit mine that you can visit for a fee, about 4 times a year. One business in Spencer also brings material down from the mine site to their store, so that would be opal miners can dig for their own opal, again for a nominal fee. A high percentage of the opal found there occurs in thin layers. As a result, most of the production goes into the making of doublets and triplets.

Other significant deposits of precious opal around the world can be found in the Czech Republic, Slovakia, Hungary, Turkey, Indonesia, Brazil (Pedro II a city in the state of Piauí), Honduras, Guatemala, Nicaragua and Ethiopia.

In late 2008, NASA announced that it had discovered opal deposits on Mars.[7]

Synthetic opal

As well as occurring naturally, opals of all varieties have been synthesized experimentally and commercially. The discovery of the ordered sphere structure of precious opal led to its synthesis by Pierre Gilson in 1974.[4] The resulting material is distinguishable from natural opal by its regularity; under magnification, the patches of color are seen to be arranged in a "lizard skin" or "chicken wire" pattern. Synthetics are further distinguished from naturals by the former's lack of fluorescence under UV light. Synthetics are also generally lower in density and are often highly porous.

Two notable producers of synthetic opal are the companies Kyocera and Inamori of Japan. Most so-called synthetics, however, are more correctly termed "imitation opal", as they contain substances not found in natural opal (e.g., plastic stabilizers). The imitation opals seen in vintage jewelry are often foiled glass, glass-based "Slocum stone", or later plastic materials.

Local atomic structure of opals

The lattice of spheres of opal that cause the interference with light are several hundred times larger than the fundamental structure of crystalline silica. As a mineraloid, there is no unit cell that describes the structure of opal. Nevertheless, opals can be roughly divided into those that show no signs of crystalline order (amorphous opal) and those that show signs of the beginning of crystalline order, commonly termed cryptocrystalline or microcrystalline opal.[8] Dehydration experiments and infrared spectroscopy have shown that most of the H2O in the formula of SiO2·nH2O of opals is present in the familiar form of clusters of molecular water. Isolated water molecules, and silanols, structures such as Si-O-H, generally form a lesser proportion of the total and can reside near the surface or in defects inside the opal.

The structure of low-pressure polymorphs of anhydrous silica consist of frameworks of fully-corner bonded tetrahedra of SiO4. The higher temperature polymorphs of silica cristobalite and tridymite are frequently the first to crystallize from amorphous anhydrous silica, and the local structures of microcrystalline opals also appear to be closer to that of cristobalite and tridymite than to quartz. The structures of tridymite and cristobalite are closely related and can be described as hexagonal and cubic close-packed layers. It is therefore possible to have intermediate structures in which the layers are not regularly stacked.

The crystal structure of crystalline α-cristobalite. Locally, the structures of some opals, opal-C, are similar to this.

Advertisements

Microcrystalline opal

Opal-CT has been interpreted as consisting of clusters of stacking of cristobalite and tridymite over very short length scales. The spheres of opal in opal-CT are themselves made up of tiny microcrystalline blades of cristobalite and tridymite. Opal-CT has occasionally been further subdivided in the literature. Water content may be as high as 10 wt%. Lussatite is a synonym. Opal-C, also called Lussatine, is interpreted as consisting of localized order of α-cristobalite with a lot of stacking disorder. Typical water content is about 1.5wt%.

Non-crystalline opal

Two broad categories of non-crystalline opals, sometimes just referred to as "opal-A", have been proposed. The first of these is opal-AG consisting of aggregated spheres of silica, with water filling the space in between. Precious opal and potch opal are generally varieties of this, the difference being in the regularity of the sizes of the spheres and their packing. The second "opal-A" is opal-AN or water-containing amorphous silica-glass. Hyalite is another name for this.

Non-crystalline silica in siliceous sediments is reported to gradually transform to opal-CT and then opal-C as a result of diagenesis, due to the increasing overburden pressure in sedimentary rocks, as some of the stacking disorder is removed.[9]

Historical superstitions

In the Middle Ages, opal was considered a stone that could provide great luck because it was believed to possess all the virtues of each gemstone whose color was represented in the color spectrum of the opal.[10] It was also said to confer the power of invisibility if wrapped in a fresh bay leaf and held in the hand.[10][11] Following the publication of Sir Walter Scott's Anne of Geierstein in 1829, however, opal acquired a less auspicious reputation. In Scott's novel, the Baroness of Arnheim wears an opal talisman with supernatural powers. When a drop of holy water falls on the talisman, the opal turns into a colorless stone and the Baroness dies soon thereafter. Due to the popularity of Scott's novel, people began to associate opals with bad luck and death.[10] Even as recently as the beginning of the 20th century, it was believed that when a Russian saw an opal among other goods offered for sale, he or she should not buy anything more since the opal was believed to embody the evil eye.[10]

Opal is considered the birthstone for people born in October or under the sign of Libra and the star stone for people born under Scorpio.

From LoveToKnow 1911

OPAL, an amorphous or non-crystalline mineral
consisting of hydrated silica,
occasionally displaying a beautiful play of colour, whence its
value as a gem-stone. It is named
from Lat. opalus, Gr. 67r&XAcov, with which may be compared
Sansk. upala, a precious stone. Opal commonly occurs in
nodular or stalactitic masses, in the cavities of volcanic rocks,
having been deposited in a gelatinous or colloidal condition. It is
inferior to quartz in hardness
(H. 5.5 to 6.5) and in density (S. G. 1.9 to 2.3), whilst it differs
also by its solubility in caustic alkalis. The proportion of water in
opal varies usually from 3 to 1 2%, and it is said that
occasionally no water can be detected, the mineral having
apparently suffered dehydration. Though normally isotropic, opal is
frequently doubly refracting, the anomaly being due to tension set up during
consolidation. The mineral when pure is transparent and colourless,
as well seen in the variety which, from its vitreous appearance,
was called by A. G. Werner hyalite (Gr. i aXos, glass), or popularly "Miiller's
glass," a name said to have been taken from its discoverer. This
pellucid opaline silica occurs as an incrustation in small
globules, and is by no means a common mineral, being chiefly found
at certain localities in Bohemia, Mexico and Colorado, U.S.A. (Cripple Creek).

The beautiful variety known as "noble" or "precious opal" owes
its value to the brilliant flashes of colour which it displays by
reflected light. The colours are not due to the presence of any
material pigment, but result from certain structural peculiarities
in the stone, perhaps from microscopic fissures or pores or from
delicate striae, but more probably from very thin lamellae of
foreign matter, or of opaline silica, having a different index of
refraction from that
of the matrix. The origin of
the colours in opal has been studied by Sir D. Brewster, Sir W.
Crookes, Lord Rayleigh and H. Behrens. In the variety known to
jewellers as "harlequin
opal," the rainbow-like
tints are flashed forth from small angular surfaces, forming a kind
of polychromatic mosaic,
whilst in other varieties the colours are disposed in broad bands
or irregular patches of comparatively large area. By moving the
stone, a brilliant succession of fiery flashes may sometimes be
obtained. The opal is usually cut with a convex surface, and, being a soft stone, should
be protected from friction
likely to produce abrasion; nor should it be exposed to sudden
alternations of temperature. The loss of water, sometimes effected
by heat, greatly impairs the colour, though moderate warmth may
improve it. According to Pliny
the opal ranked next in value to the emerald, and he relates that the rich Roman
senator Nonius was exiled by Mark
Antony for sake of his magnificent opal, as large as a hazelnut. The opal, on account of its unique characters,
has been the subject of remarkable superstition, and even in modern
times has often been regarded as an unlucky stone, but in recent
years it has regained popular favour and is now when fine, among
the most highly valued gem-stones.

Precious opal is a mineral of very limited distribution. Though
ancient writers state that it was brought from India, and fine stones are still called in trade
"Oriental opal," its occurrence is not known in the East. The
finest opals seem to have been always obtained from Hungary, where the mineral
occurs, associated with much common opal, in nests in an altered
andesitic rock. The fine opals occur only at the Dubnyik mine, near
the village of VOrOsvagas (Czerwenitza). The workings have been
carried on for centuries in the mountains near Eperjes, and some remarkable stones from this
locality are preserved in the Imperial Natural History Museum in Vienna, including an uncut
specimen weighing about 3000 carats. Precious opal is found also in
Honduras, especially in trachyte near Gracias a Dios;
and in Mexico, where it occurs in a porphyritic rock at Esperanza
in the state of Queretaro. A remarkable kind of opal, of
yellow or hyacinth-red
colour, occurs in trachytic porphyry at Zimapan in Hidalgo,
Mexico, and is known as "fire-opal." This variety is not only
cut en cabochon but is also faceted. Fire-opal is
sometimes called "girasol." Much precious opal is worked in Australia. In Queensland it is found
lining cracks in nodules of brown ironstone in the DesertSandstone, a rock of Upper Cretaceous
age, and is distributed over a wide area near the Barcoo river.
Bulla Creek is a well-known locality. The layer of opal, when too
thin to be cut with a convex surface, is used for inlaid work or is
carved into cameos which show to much advantage against the
dark-brown matrix. The matrix penetrated by veins and spots of opal, and perhaps heightened
in colour artificially, has been called "black opal"; but true
black opal occurs in New South Wales. The "root of opal"
consists of the mineral disseminated through the matrix. In New
South Wales precious opal was
accidentally discovered in 1889, and is now largely worked at White
Cliffs, Yungnulgra county, where it is found in nodules and seams
in a siliceous rock of the Upper Cretaceous series. It is notable
that the opal sometimes replaces shells and even reptilian bones,
whilst curious pseudomorphs, known as "pineapple opal," show the opal in the form
of aggregated crystals, perhaps of gypsum, gaylussite or glauberite.

"Common opal" is the name generally applied to the varieties
which exhibit no beauty of colour, and may be nearly opaque. It is
frequently found in the vesicular lavas of the N.E. of Ireland, the west of Scotland, the Faroe Isles and
Iceland. When of milky-white
colour it is known as "milk opal";
when of resinous and waxy appearance as "resin opal"; if banded it is called "agate opal"; a green variety is
termed "prase opal"; a dark red, ferruginous variety "jaspar opal";
whilst "rose opal" is a beautiful
pink mineral, coloured with
organic matter, found at Quincy, near Mehun-sur-Yevre, in France. A brown or grey
concretionary opal from Tertiary shakes at Menilmontant, near Paris, is known as menilite or "liver opal." A dull opaque form of
opal, with a fracture imperfectly conchoidal, is called
"semi-opal"; whilst the opal which not infrequently forms the
mineralizing substance of fossil wood passes as "wood opal." The
name hydrophane is applied to a porous opal, perhaps partially
dehydrated, which is almost opaque when dry but becomes more or
less transparent when immersed in water. It has been sometimes sold
in America as "magic stone." Cacholong is another
kind of porous opal with a lustre rather like that of mother-of-pearl, said to have been named from
the Cach river in Bokhara,
but the word is probably of Tatar origin.

Opaline silica is frequently deposited from hot siliceous
springs, often in cauliflower-like masses, and is known as
geyserite. This occurs in Iceland, New Zealand and the Yellowstone National Park.
The fiorite from the hot
springs of Santa Fiora, in Tuscany, is opaline silica, with a rather
pearly lustre. A variety containing an exceptionally small
proportion of water, obtained from the Yellowstone Park, was named
pealite, after the chemist A. C. Peale. The siliceous deposits from
springs, often due to organic agencies, are known generally as
"siliceous sinter" or, if very
loose in texture, as "siliceous tuff." Opaline silica forms the material of many
organic structures, like the frustules of diatoms and the tests of
radiolarians, which may accumulate as deposits of tripoli, and be used for
polishing purposes. (F. W. R.*)

English

Proper
noun

Perhaps to console herself for the bad luck she had already
suffered, in a back corner of South Sherbrooke Township - or
perhaps to make up, ahead of time, for a lack of motherly feelings
- she gave the girls the fanciest names she could think of:
Opal Violet, Dawn Rose, and Bonnie Hope.

(Australian) A type
of petrol made by the BP
company designed to be unable to used for petrol sniffing.
(Reference: BP article on Opal [1].)